In general, a hydraulic circuit on an industrial vehicle or a construction vehicle such as a power shovel incorporates a filtering device having filters. The filtering device has a device for indicating the degree of contamination which displays the timing of renewal or cleaning of filters or timing of renewal of contaminated oil, thereby preventing occurrence of troubles in the hydraulic system.
The contamination degree indicating device used in this type of hydraulic circuit has to cope with a wide distribution of the sizes of contaminant particles in the oil, from substantially 0 to several hundreds of .mu.m. Such a hydraulic circuit employs one or a plurality of filters of the same mesh, e.g., a capability to trap particles of 20 to 30 .mu.m, each filter being provided with a contamination degree indicating device capable of indicating the degree of clogging of the filter. As shown in FIG. 9, oil returning from an actuator or a hydraulic motor flows to an oil tank 153 through a return passage 151 via a filter 152. Contaminants in the oil, therefore, are trapped by the filter 152. As the amount of contaminants accumulated on the filter increases, the clogging of the filter becomes heavier so that a greater flow resistance is produced by the filter, with the result that the oil pressure rises at the inlet side of the filter. In consequence, a bypass valve 154 is opened to cause the oil to flow into the oil tank 153 bypassing the filter 152. Thus, when the amount of contaminants in the oil increases to cause a heavier clogging of the filter, the pressure differential between the inlet and outlet sides of the filter is increased correspondingly, so that the degree of clogging of the filter can be known through indication of the level of the pressure differential. A pressure sensor 155 is arranged to operate when the pressure differential between the inlet side and the outlet side of the filter reaches a predetermined level, and an indicating device 156 performs indication in response to a signal from the pressure sensor 155. The indication is usually conducted in advance of the opening of the bypass valve 154. The user replaces or cleans the filter or renews the contaminated oil upon recognition of the indication.
In the conventional system described above, the filter traps all the contaminant particles of sizes which widely vary between 20 or 30 .mu.m to several hundreds of .mu.m, and the indication is conducted when the pressure differential across the filter has reached a predetermined level. Thus, it is impossible to discriminate between the sizes of the contaminant particles. Namely, when particles of comparatively large sizes are being generated and contained in the oil, the indication device does not operate while the pressure differential is still low, often resulting in a breakdown of the hydraulic system.
Accordingly, an object of the present invention is to provide a device for indicating the degree of contamination of a hydraulic circuit and a method of judging the degree of contamination, wherein the states of clogging of filters are indicated at proper timings according to the sizes of the contaminant particles in the oil, thereby preventing breakdown of the hydraulic system.